With conventional domestic gas ovens, a blue flame atmospheric burner is typically positioned in a chamber below the oven cavity. The best efficiency has been achieved by providing communicating apertures in the floor of the cavity so that the combustion vapors can pass from the chamber directly into the cavity by natural convection. Furthermore, it has been common to position an additional burner such as a radiant burner in the cavity for broiling. The burners in these described environments are located in large volume atmospheric combustion chambers. Accordingly, almost all conventional blue flame atmospheric burners can be used with favorable results in these applications.
The introduction into the oven of apparatus for energizing the cavity with microwave energy so as to provide a combination microwave gas oven alters the conventional gas technology approach. More specifically, it was found desirable to position the magnetron, power supply, and waveguide coupling underneath the oven cavity in the chamber previously occupied by the gas burner in a conventional gas oven. Therefore, the burner was positioned back of and underneath the oven cavity. The volume to be allocated to the burner in this configuration was further limited by the requirements of isolating the microwave components and oven exterior surfaces from high temperatures; in addition to the damage that could be caused to the microwave components, American National Standard Institute standards regarding fire prevention and burning hazard had to be satisfied. Furthermore, a system of forced convection was preferable because among other reasons, the combustion vapors from the burner at the rear of the oven were to be transferred into the cavity for enhanced efficiency. Also, because of the microwave energy within the cavity, it was not desirable to position a radiant burner therein. The combination of the above described design parameters meant that it was desirable to have a gas burner that operated in a relatively small volume having an induced draft. Also, with the induced draft or negative pressure above the burner, it was desirable to restrict the secondary combustion air so as to improve efficiency.
A variety of conventional atmospheric blue flame burners were installed in the environment described above. However, good flame stability in the negative pressure without the sound of combustion noise was difficult to attain.
Infrared burners, such as, for example, one having a very large port covered by perforated steel or wire mesh layers, were tried as one approach. The flame characteristics were improved over other conventional burners in the negative pressure by the reduced port loading, but the infrared radiant heat was not very efficient for a forced air convection system. Furthermore, the mesh raised to extremely high temperatures in the oven self-clean mode.
Another approach such as described in my U.S. application Ser. No. 4,007, filed Jan. 16, 1979, assigned to the same assignee herein, and hereby incorporated by reference, is a ribbon burner. The tight flame on the top of the ribbon burner and the secondary air flow under induced draft parallel but spaced from the direction of the gas mixture resulted in a relatively quiet flame with good flame characteristics. A longitudinal gap was provided between two ribbon sections to provide improved secondary air entry to the combustion chamber. However, the ribbon burner was substantially more expensive to fabricate than conventional burners. Also, because of the increased burner temperature caused by the tight flame, the burner had to be fabricated of an expensive material such as stainless steel.